This research is concerned with the molecular analysis of the arrangement and regulation of gene systems in Drosophila melanogaster. Primary emphasis in the future will be placed on the genetic regulatory hierarchies triggered by the steroid hormone, beta-ecdysone, and in particular on the genes of the ecdysone-triggered hierarchy in the larval salivary gland, which was first defined by the changes in the pattern of puffs observed in the polytene chromosomes of these glands upon their exposure to this molting hormone. Many of the genes in this hierarchy have been isolated by molecular cloning and analysis has progressed in some cases to the nucleotide-sequence level. We are interested in learning how the steroid hormone modulates the expression of the upper members of the hierarchy and how the products of these genes in turn modulate the expression of the next rank in the hierarchy. In addition, we are studying the arrangement and expression of the five histone genes which are contained in repeated units that are clustered at one locus occupying five polytene chromosome bands and are arranged in multiple tandem arrays separated by array spacers consisting of DNA sequences not found in the repeating units. We are also studying a particular aspect of the heat-shock response--namely, the inhibition of the processing by which the 135-nucleotide precursor of the 5S RNA is converted to the 5S RNA. At present, we have an in vitro system whereby the inhibitory activity found in extracts of heat-shocked cells can be assayed and wish to define the inhibitors as well as the enzymes of this RNA processing.